The present invention relates to an information recording medium, and more particularly, to an optical information recording medium, which is capable of reading out or recording with a high recording density, and which has a high reliability in repeating recording and regeneration operations.
Conventionally, compact disks (CD), laser disks (LD), and the like are used widely as optical information recording media. Currently, a DVD, which has seven times the recording density of a CD, has come into practical use. The DVD is being developed as an erasable recording-regenerating medium in addition to a read only medium (DVD-ROM), wherein information is directly written onto the substrate. Furthermore, the practical use of a DVD as a RAM for a computer presently is under investigation.
With the DVD, high density recording can be achieved by using a laser having a shorter wave length, such as 650 nm, than the laser used for a CD (wave length approximately 780 nm). However, in order to handle a large amount of information, such as computer graphics and the like, it is necessary to achieve a higher recording density, such as 1.5 to 2 times that of the conventional high density recording. In order to achieve such a high recording density, a semiconductor laser of green to blue color having a shorter wave length (wave length 520-410 nm) than ever is under development.
As another means to achieve a higher recording density, a super resolution film can be employed. The super resolution film is a thin film formed at a lower plane of the recording medium, with which a high recording density can be achieved by the fact that it is able to decrease the size of the beam spot of the incident light passing through the film.
One of the mechanisms of the super resolution effect is an absorption-saturation phenomenon, which is a phenomenon utilizing non-linear optical characteristics of the super resolution film such that the film allows light having a larger intensity than the amount of its absorption-saturation to pass through the film and absorbs any light having an intensity less than the amount of its absorption-saturation. The spatial intensity of a laser beam utilized in reading and writing has a Gaussian distribution. Therefore, when the laser light beam passes through the super resolution film, the laser light in the lower end portion of the Gaussian distribution, where the intensity is low, is absorbed by the film, and the laser light in the middle portion of the Gaussian distribution, where the intensity is high, passes through the film. Accordingly, the diameter of the laser beam is reduced as it passes through the super resolution film.
An organic thin film made of a material in the phthalocyanine group, as disclosed in JP-A-8-96412 (1996), chalcogenide, fine particles of a compound semiconductor, and the like are known at the present as materials which may be used for the super resolution film described above. Additionally, trials to use some organic materials, such as thermochromic materials of the type disclosed in JP-A-6-162564 (1994), and photochromic materials of the type disclosed in JP-A-6-267078 (1994), as the super resolution film have been carried out.
However, the above-mentioned materials have problems in reliability and productivity. That is, there has been a concern about gradual deterioration of the organic thin film after repeated recording and regenerating operations, because the energy density of a laser beam is locally increased significantly during the recording and regenerating operations. Therefore, a sufficient guarantee period for the recording and regenerating operations is scarcely obtained under a severe condition of use, wherein the recording and regenerating operations are performed frequently, such as when the disk is used as a RAM and the like for computers.
On the other hand, chalcogenide is chemically unstable, and so a long guarantee period can not be obtained for this material, and the fine particles of a compound semiconductor provide difficulties during the production process.